1. Area of the Invention
This invention relates to digital bus termination and more importantly relates to matched impedance termination.
2. Description of the Prior Art
As bus speeds such as those used in computers have increased in speed of operation, attention to RF analog design techniques in designing the buses have become of increasingly greater importance. While at bus speeds of 10 MHZ or below, such analog techniques may be ignored, at higher bus speeds, for example, impedance matching becomes a significant consideration. Tests have established that at bus speeds beyond 10 MHZ, if significant impedance mismatching occurs, reflections on the bus may cause data or address errors on the bus with digital ones being detected as zeros or digital zeros being detected as ones.
To overcome these problems, a number of design changes have been made. First, in dedicated high speed backplane buses where design parameters are well controlled, precision resistors may be used for proper termination. However, in conventional personal computers such as IBM PC compatibles and MAC compatibles, such designs are not readily practical.
One attempt at impedance matching, the original SCSI specification (now called SCSI-1) specified a 132.OMEGA. resistor formed by two resistors, one coupled from the bus to Vcc and the other coupled from the bus to ground. However, practical considerations caused problems. For example, typical commercially available resistor are not accurate enough to provide an accurate 132.OMEGA. impedance. As a result, impedance mismatching occurred. In addition, such resistor terminators generally had to be re-installed as a plug in module as each device was added to the SCSI bus. However, since often the person adding a SCSI device is not a skilled computer technician, but may be an ordinary consumer, problems arise due to improper installation of the terminators.
A still further problem with such terminators is that as more devices are added to the bus, the impedance characteristics of the bus changed. This caused impedance mismatching that could not be readily overcome by matching with fixed resistor terminators.
Therefore, to overcome some of these drawbacks, terminator standards were developed that had specified voltage and current characteristics. For example, the SCSI-2 standard requires a specific voltage current profile. For voltages above 2.85 volts, the terminator is supposed to appear as a voltage source of 2.85 volts. For voltages below that level, the terminator is suppose to appear as a current source of 24 mA driving a 110.OMEGA. of resistor. However, graphing the voltage current curve of that specification shows that the voltage current curve has a ninety degree angle, which at best can only be approximated by a Boulay terminator or a transistor current source. Still further, the 110.OMEGA. resistor is also not easily produced precisely in circuitry leading to impedance mismatching. To attenuate reflections resulting from such mismatching, however, the circuits are also commonly provided with over and under voltage clamps that limit the voltage excursions. This creates increased cost and complexity.
An additional drawback of terminators according to the SCSI standard is that they draw considerable excess current. In an addressing scheme with for example thirty two address lines, sixty four bits of data lines, plus various other chip enable and read and write enable lines, over 100 lines may need termination. Using the SCSI-2 standard of 24 mA for over one hundred lines means that termination will require 2.4 amps of current. For five volt circuits this means that the termination will require ten watts. Ten watts of power, however, in a portable computer such as a notebook size computer will waste battery power and contribute to excessive heating of the unit.
A further problem with such terminators arises with the inclusion of terminators on computer buses where the number of devices might change. Examples include high speed computer buses such as used for system memory, PCI buses or video cards memory buses. For example, many PC's are sold today with either 8 MB or 16 MB of memory installed using 30 or 70 pin Single In-line Memory Modules (SIMM's). Those SIMM's provide for ready expandability of system or video memory such as by installing additional SIMM's or replacing the original SIMM's with SIMM's containing more memory. Often users of those PC's find it necessary to increase the amount of memory to 32 MB or more to run applications or run multiple applications efficiently based upon software upgrades or operating system upgrades. Still further, many video boards are sold with only half or less of the maximum memory of the board installed to hold down cost of the system. When the typical consumer adds more SIMM's to the system or adds more memory to the memory card, the impedance of the address, data, chip enable and select lines change. Similar effects can be found with video cards where more memory is added to the video cards. It is difficult to properly terminate the bus as the bus impedances will vary with the number of components. Use of plug-in terminators such as for SCSI-1 have drawbacks as consumers frequently install the terminators incorrectly.
Therefore, it is a first object of the invention to provide a precise impedance match for the characteristics of the bus irrespective of the number of devices connected to the bus. It is a second object of the invention to provide such impedance matching that may be readily used by the home consumer. It is a third object of the invention to permit the impedance of the bus to vary with the number of device coupled to the bus. It is yet another object of this invention to provide such termination with minimal power consumption. It is still yet another object of this invention to provide such termination with readily producible circuits and minimizing expensive components such as precision resistors.